In this work we present the improvement on the compositional and structural properties of silicon dioxide films deposited by direct plasma enhanced chemical vapor deposition (DPECVD) from gaseous mixtures of N2O and SiH4. The studied films were deposited at substrate temperatures of 320°C, SiH4 flow of 3 seem, rf power density of 250 mW/cm2 and always keeping the residence time on the minimum value permitted by the deposition system. In order to determine the structural characteristics, we use the FTIR and Ellipsometry techniques. It is known that the frequency of the stretching vibration of the Si-O bond is related with the bond angle and with the stoichiometry of the material. So in this work we analyze the dependence of the vibration frequency of the Si-O peak with the deposition conditions as well as its evolution with thermal annealing and the thermal relaxation of the films.